1. Field of the Invention
The present invention relates to a method for forming boron nitride coatings. More specifically, the present invention relates to forming a boron nitride film having a low amount of impurities on a substrate.
2. Description of Related Art
Chemical vapor deposition (CVD) would be particularly attractive for BN film formation, and considerable effort has occurred in this direction using various gas mixtures: B.sub.2 H.sub.6 and NH.sub.3 (S. Hirano et al, J. Am. Ceram. Soc. 72. 66(1989)); BC1.sub.6 and NH.sub.3 (S. Motojima et al, Thin solid Films, 88, 269(1982)); B.sub.10 H.sub.14 and NH.sub.3 (K. Nakamura, J. Electrochem. Soc., 132, 1757(1985)); direct pyrolysis of borazine (S. Hirano et al, supra); and more complex BN containing molecules (W. Schmolla et al, Solid-State Electronics, 26 931(1983)). However, these reactions are highly complex and the resulting films strongly dependent on the gas composition, the temperature and reactions occurring at the wall (S. Hirano et al, supra).
A potentially interesting precursor for BN film production is the borazine (B.sub.3 N.sub.3 H.sub.6) molecule. Borazine is isostructural and isoelectron with benzene, and contains the exact B/N ratio required for BN with only hydrogen as a potential impurity. The fact that borazine contains the same 6-membered ring structure as the hexagonal phase of boron nitride, h--BN, is not important since the formation of h--BN from borazine requires ring opening reactions.
There are a number of applications where it would be very desirable to use a thin coating of BN as a protective coating. One area is in the use of materials such as BN as a passivation layer for high-frequency electronic devices based on materials such as GaAs. Schmolla et al (supra) pointed out that BN is an excellent candidate for device application but that the following deposition conditions are dictated by the semiconductor: 1) low substrate temperature, 2) low heat of formation, and 3) low energy incident particles. These requirements would tend to eliminate ion beam or plasma techniques and place strong restrictions on normal chemical methods of film formation given the problems with residence time of surface species. In general, one increases the rate of surface chemical reactions by increasing the temperature, but the same increase in temperature leads to a decreased residence time on the surface for the reacting species. The result of this competition between increased reaction rate and decreased residence time gives a general phenomenon in surface chemical reactions of maximum in the overall rate vs. temperature.
It has been shown specifically for BN that chemical vapor deposition (CVD) requires a temperature of over 800.degree. C. for the production of stoichiometric BN, and moreover, that a factor of 5 increase occurs in both the N/B ratio and the deposition rate in going from 350.degree. C. to 800.degree. C; above 800.degree. C. both decrease with increasing temperature (Nakamura et al, supra). In a particularly clear example of the composition and temperature dependence, when B.sub.10 H.sub.14 and NH.sub.3 were used, a ratio of 20:1 for NH.sub.3 :B.sub.10 H.sub.14 is required for a N:B ratio of 1:1 in the resulting film (Nakamura et al, supra). In general, this need for a high substrate temperature is a characteristic of CVD processes in which the reaction occurs on a heated substrate. A basic problem is to increase the chemical activity of the reactant species without increasing the substrate temperature. In normal CVD the increased chemical activity is produced by the temperature of the gas; this also disadvantageously heats the sample.
A method for catalytic chemical vapor deposition which entails deposition at a substrate temperature between room temperature and 500.degree. C. with decomposition of a source gas which is brought into contact with a catalyst superheated at 800.degree.-2000.degree. C. is disclosed by Matsumura (Chem. Abs. 109(10):83940z). It is disclosed that amorphous silicon and silicon nitride films are among the films which may be formed by this method. Other relevant disclosures by Matsumura include H. Matsumura, Jap. J. Appl. Phys. 25, L949(l986); H. Matsumura et al, J. Appl. Phys. 64, 6505(1988); H. Matsumura, J. Appl. Phys., 65, 4396(1989); H. Matsumura, J. Appl. Phys., 28, 2157(1989); and H. Matsumura, J.Appl. Phys., 66, 3612(1989). All of these disclosures fail to disclose a method for forming a boron nitride film on a substrate.